JPH08160246A - Fiber fusion stretching type polarization beam splitter - Google Patents

Abstract

PURPOSE: To make it possible to lessen dependency on wavelength to such an extent that has to hindrance for practicable use by setting the stretching amount of a fused part at a specific range. CONSTITUTION: Optical fibers OPF1 and OPF2 are fused to each other in the heated position in the state that a flame exists nearest the optical fibers OPF1 and OPF2 . These fibers are then stretched by the tension of a stretching machine 13. A branching ratio is measured with a branching ratio measuring instrument 23 during stretching and the state that the incident, for example, Y polarized light on the optical fiber OPF1 from a light source 19 is branched to the other optical fiber OPF2 100% or nearly in the same state as it, is detected. The state that the polarized light is branched 100% to another optical fiber different from the optical fiber on which the light described above is made incident or the state nearly the same as it gives a detection signal to a controller 24 in the first time, thereby stopping the stretching operation of a stretching machine 13. The stretching amount of this time is about 7mm, the wavelength λ1 of the y polarized light from the light source 19 is 1.55μm and the stretching amount is about 4500 times the wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber fusion-stretched polarization beam splitter which can be used as, for example, a beam splitter of a device for detecting a fault in an optical fiber.

[0002]

2. Description of the Related Art A bulk polarization beam splitter is generally well known as a means for separating X-polarized light and Y-polarized light having orthogonal polarization planes. FIG. 8 shows a schematic configuration of an optical fiber fault point detection device using a bulk polarization beam splitter. In such a bulk type polarization beam splitter, a laser oscillator 1, a bulk type polarization beam splitter 2,
The condenser lens 3, the fiber 4 to be measured, and the light receiver 5 must be fixed by some method (for example, an adhesive), but since this fixing method usually changes with temperature or changes with time, it can be used as a polarization beam splitter. There is an inconvenience that the performance also fluctuates.

From the above, a polarization beam splitter made of an optical fiber is described in, for example, IEICE Technical Report V.
OL. 85, NO. 18 (P19-24). In this paper, it is reported that polarization beam splitting characteristics can be obtained by fusion-spreading two polarization-maintaining optical fibers with each other, and making the fusion-spreading amount larger than that in the case of making an ordinary optical branching device. ing.

FIG. 9 shows the structure of the optical fiber fusion-stretched polarization beam splitter proposed in this paper. As shown in the sectional structure of FIG. 10, the optical fibers OPF ₁ and OPF ₂ are polarization-maintaining optical fibers having a stress imparting portion 12 at a position facing each other 180 ° in the clad 10 with the core 11 interposed therebetween. Part of the optical fibers OPF ₁ and OPF ₂ are heated in a state where the lines connecting the stress applying portions 12 are arranged in parallel, and the side edges are brought into contact and fused. It is said that this fused portion is stretched to obtain a desired polarized beam splitting characteristic.

That is, the polarization beam splitting characteristic means the light E _X having a plane of polarization from the arm A in the X-axis direction and the Y-axis direction,
When E _Y is incident, a light E _Y having a plane of polarization in the Y-axis direction (hereinafter referred to as y-polarized light) is obtained from the arm C, and a light E _X having a plane of polarization in the X-axis direction (hereinafter x polarization) is obtained from the arm D. (Referred to as wave light). The results of trial production by the author of this paper are shown in FIGS. 11 and 12. FIG. 11 shows the branching ratio wavelength characteristics when the fusion drawing amount is 9 mm. FIG. 12 shows the branching ratio wavelength characteristics when the fusion drawing amount is 20 mm.

The branching ratio wavelength characteristic is, for example, a change in intensity of polarized light emitted from arm C or arm D when the wavelength of light incident from arm A is changed from 0.8 to 1.5 μm, that is, Refers to the change in branching ratio between arms A and C or A and D. In the figure, the solid line indicates the intensity of x-polarized light, and the broken line indicates the intensity of y-polarized light. In the example of FIG. 11, the phase difference in which the x-polarized light and the y-polarized light change is close to each other. From this, x
It shows that the polarized light and the y-polarized light cannot be separated sufficiently.

In FIG. 12, the closer the wavelength is to the long wavelength side, the more the phases of the x-polarized light and the y-polarized light have opposite phases. For example, at the wavelength of 1.4 μm, the branching ratio of the y-polarized light is almost 100%. With respect to the x-polarized light, the branching ratio is almost 0%, which shows the characteristic that only the y-polarized light can be extracted. At this time, the other arm is in a state where only the x-polarized light can be extracted.

As is clear from the measurement results shown in FIG. 12, it was confirmed that the polarization beam splitter can be used practically if the fusion and stretching amount is long and the wavelength of light is limited to a certain wavelength.

[0009]

When the polarization beam splitter having the branching ratio wavelength characteristic shown in FIG. 12 is put into practical use,
There is an inconvenience that the wavelength of light that can be beam-split is limited. Further, as is clear from the characteristics shown in FIG. 12, it exhibits a characteristic in which the branching ratio largely changes with a change in wavelength (hereinafter, this characteristic is referred to as a wavelength-dependent method). As a result, even if the wavelength is limited and put to practical use, if the wavelength changes even slightly, the branching ratio greatly changes due to the wavelength change.
Since the intensity of polarized light also fluctuates, if it is used in the optical fiber fault point detection device shown in FIG. 8, for example, there is a drawback that it hinders the measurement of the fault point.

A report aimed at reducing the wavelength dependence is the 2nd Workshop on Optical.
This is done in WOFS2-13 at Sensors. In this paper, the wavelength dependence of the optical fiber fusion drawing type beam splitter is reduced by increasing the stress birefringence of the polarization-maintaining optical fiber used or by increasing the refractive index difference between the stress applying part 12 and the cladding 10. It is reported that it can be done.

However, even if the wavelength dependence is reduced by the method reported in this paper, nothing is described about how much the wavelength dependence is reduced. Therefore, the present inventor prototyped a polarization beam splitter using the method reported in this paper, and confirmed its reduction effect. FIG.
While the period T of the branching ratio wavelength characteristic reported in the above-mentioned paper shown in FIG. 2 is 120 nm, the present inventor made a prototype under the best conditions of the method proposed in the latter paper.
The cycle T at which the branching ratio changes was improved to 200 nm. That is, the birefringence index B is currently the highest (B = 4.4 × 10
^-4 ) A prototype was made using a polarization-maintaining fiber, and the result was that the period T of the coupling ratio wavelength characteristic was improved to 200 nm, and it can be said that the wavelength dependence is still high.

An object of the present invention is to propose an optical fiber fusion-stretched polarization beam splitter capable of reducing the wavelength dependence to the extent that there is no practical problem, and a manufacturing method thereof.

[0013]

SUMMARY OF THE INVENTION A method for manufacturing an optical fiber fusion-stretched polarization beam splitter according to the present invention heats a polarization-maintaining fiber at a temperature lower than the conventional melting temperature of the optical fiber and forms a part of a side edge. Is fused and stretched in a highly viscous state so as to increase the internal stress of the fused portion.

Further, in the present invention, in the process of executing the fusion drawing, the Y-axis polarized light is made incident from one end of one optical fiber, and a branching ratio measuring device is connected to both ends of the optical fiber to branch the light. The fusion drawing is performed while monitoring the change in ratio. In the process of extending the fusion-bonded portion, 100 of the polarized light incident from a fiber other than the fiber into which the polarized light is incident.
The stretching is stopped at the time when the polarized light close to% is emitted.

The characteristic feature of the fiber fusion-spreading type polarization beam splitter thus produced is that the branching ratio is 10
On the shorter wavelength side than the wavelength at which 0: 0 (%) is achieved, the branching ratio does not change and exhibits a flat characteristic, and on the long wavelength side of that wavelength, a sufficiently long wavelength period is provided. Therefore, the change of the branching ratio becomes gradual with respect to the change of the wavelength, and the wavelength dependence can be sufficiently reduced.

The fiber fusion-spreading type polarization beam splitter according to the present invention is characterized in that the stretching amount of the stretching portion achieves a branching ratio of 100: 0 (%), which is about 60 of the wavelength of light.
The point is that the size is less than 00 times. By the way, in the optical fiber fusion stretched polarization beam splitter reported in the previously published paper, the stretched amount (20 mm) is 10%.
Approximately 14 x of wavelength (1.4 μm) that realizes 0: 0 (%)
It is a little over 10 ³ times.

The fact that a predetermined polarization separation characteristic can be obtained with a short amount of stretching as in the present invention means that the birefringence of this portion is increased due to the increase of the internal stress in the fusion-stretched portion. This means that the polarization separation characteristics with low frequency dependence can be obtained by increasing the birefringence of the fused and stretched portion. This provides a polarization splitting characteristic that is even lower than the method proposed in the paper (latter) that uses an optical fiber with high birefringence of the optical fiber itself.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a diagram for explaining the manufacturing method of the present invention. The optical fiber fusion splicing and drawing apparatus shown in FIG. 1 has exactly the same configuration as that disclosed in Japanese Patent Laid-Open No. 5-307128. In this embodiment, a case where two polarization-maintaining fibers are fused and drawn will be described. Two polarization-maintaining fibers (hereinafter, simply referred to as optical fibers) OPF ₁ and OPF ₂ are arranged in parallel,
One end side is fixed by the fixed clamp 14, and the other end side is supported by the stretching machine 13. The drawing machine 13 applies a drawing force to the optical fibers OPF ₁ and OPF ₂ by the motor 15. Optical fiber O
The PF ₁ and OPF ₂ are given a single polarized light from the light source 19 in advance, and the stress applying unit 1 is measured by measuring the polarization axis direction of the transmitted light.
The arrangement direction of 2 (see FIG. 10) is specified, and the arrangement directions of the stress applying portions 12 of the _two optical fibers OPF ₁ and OPF ₂ are aligned in parallel postures.

The light source 19 emits X-axis polarized light or Y-axis polarized light.
Optical fiber, OPF in the example shown ₁Enter into one end of
Shoot. Optical fiber OPF₁And OPF₂On the other end of
Is arranged so that the photodetectors 21 and 22 are optically coupled and
The detection signal is input to the branching ratio measuring device 23, and
It is configured so that the ratio can be measured. Optical fiber OPF
₁And OPF₂Of the heating means 16 facing the planned fusion position of
17 are arranged. The heating means 16 and 17 are, for example, gas
A heater or an electric heater can be used. In the figure
The example shows the case where a gas burner is used. Heating means 16 and
17 is mounted on a moving stage 18 as shown in FIG.
The optical fiber OPF is controlled by the controller 24.₁And OPF
₂It is operated to move toward and away from
It That is, initially, the heating means 16 and 17 have the optical fiber O
PF₁And OPF₂It is located in a position away from. Fused
Along with the start operation, the heating means 16 and 17 cause the optical fiber OP
F₁And OPF₂Start moving in the direction of approaching. Heating means
If 16 and 17 are gas burners, give them to the gas burner.
Gas flow rate is maintained at a constant value, and thermal power, that is, temperature is kept constant
Keep at the value. The controller 24 has the tip of the gas burner flame in advance.
End and optical fiber OPF₁And OPF₂The closest distance between
Set the law.

Optical fiber OPF₁And OPF₂Completely melt
Close to (the viscosity is low enough to stretch without resistance)
If it melts, the tip of the flame and the optical fiber OPF₁And O
PF ₂The closest distance between and was 2 mm. Conventional technique
This distance was set for the prototype described in the section on surgery. Against this
However, in this invention, the tip of the flame and the optical fiber OPF₁, OP
F₂The closest distance between them was set to 2.3 mm. Closest
Optical fiber OPF₁
And OPF₂Is in a semi-molten state without reaching a state close to complete melting.
Optical fiber OPF in the state (high viscosity state)₁And OPF₂
Are fused and in the semi-molten state by the tension of the stretching machine 13.
It is stretched. Heating means 16 and 17 at the position where the flame is closest
Will remain stationary for about 1 second, and after the stationary
Fiber OPF₁And OPF ₂Move away from,
Move to a position where there is no effect of flame heat and stop.

The closest distance of the flame must be changed to the closer direction when the temperature of the gas burner becomes lower (by reducing the gas supply amount). Further, in the case of taking a long stationary time at the closest position, it is necessary to change the closest distance to the direction of separating. Example 1 10 minutes per minute from a gas burner with a nozzle diameter of 0.5 mmφ
Gas of cc was caused to flow out and ignited to form a flame of about 5 mmφ. The optical fiber OPF with this flame at a speed of 1 cm per second
_1. Close to both sides of OPF ₂ and stop at the position with the closest distance of 2 mm. When 0.8 seconds had elapsed, the flame was moved again at a speed of 1 cm per second, and the optical fibers OPF ₁ , O
Separated from PF ₂ .

The flame in a state that is closest to the optical fiber OPF _1, OPF _2, optical fibers OPF ₁ and OPF ₂ is fused to each other at the heating position, it is stretched by the tension of the drawing machine 13. The branching ratio measuring device 23 measures the branching ratio during the extension, and a state in which, for example, Y-polarized light incident on the optical fiber OPF ₁ from the light source 19 is branched to the other optical fiber OPF ₂ in a state of 100% or close thereto. To detect. Polarized light is transmitted to another optical fiber different from the incident optical fiber.
% Or a state close to this state, the detection signal was given to the controller 24 at the first time, and the stretching operation of the stretching machine 13 was stopped. At this time, the stretching amount was 7 mm, and the wavelength λ ₁ of the y-polarized light incident from the light source 19 was 1.55 μm. Therefore, the stretching amount is about 4500 times the wavelength.

FIG. 3 shows the branching ratio characteristics of the optical fiber fusion-stretched polarization beam splitter manufactured as described above.
The dotted line shown in FIG. 3 indicates the branching characteristic of Y polarized light. Light source 19
From the above, Y polarized light of wavelength λ ₁ is applied to the optical fiber OPF ₁ and the Y polarized light is branched to the optical fiber OPF ₂ by nearly 100%. At this time, when the X polarized light is incident on the optical fiber OPF ₁ from the light source 19, the X polarized light is not emitted from the optical fiber OPF ₂ at all. That is, in this state, the X polarized light is the same optical fiber OPF as the incident optical fiber.
It is emitted from the end of ₁ and exhibits polarization beam splitting characteristics.

The characteristic of this branching ratio characteristic is that a branching ratio of 1: 0 (%) is realized at the wavelength λ ₁ of polarized light provided from the light source 19 for measuring the branching ratio at the time of fusion drawing, and its wavelength λ.
A point where the branching ratio converges to a constant value on the shorter wavelength side than ₁ and a point where the branching ratio of either Y polarized light or X polarized light changes gradually from the convergent value on the short wavelength side toward the long wavelength side. Is. By utilizing this gently changing characteristic portion, the wavelength dependence can be sufficiently reduced. Since the wavelength dependence of the branching ratio characteristic is low, it is considered that the internal stress in the fused portion of the fused extension of the beam splitter made in this example was increased and the birefringence was enhanced. That is, the cross section of the fusion-stretched portion can be regarded as a shape in which the clad 10 and the stress applying portion 12 are largely ovalized, as shown in FIG. It can be considered that the birefringence in the clad is increased by making the cross-sectional shapes of the clad 10 and the stress applying portion 12 largely oval, and as a result, the branching characteristic as shown in FIG. 3 is obtained. Example 2 10 minutes per minute from a gas burner with a nozzle diameter of 0.5 mmφ
A gas of cc was caused to flow out and ignited to form a flame of about 5 mmφ. This flame is applied to the optical fiber OPF at a speed of 1 cm for 1 second.
₁ and OPF ₂ were approached on both sides and stopped at the position where the closest distance was 2.3 mm.

When one second has passed, the flame is moved again at a speed of 1 cm per second, and the optical fibers OPF ₁ and OPF ₁ are moved.
Pulled away from ₂ . Flame is optical fiber OPF ₁ , OPF ₂
With the optical fibers OPF ₁ and OP
F ₂ is fused to each other at the heating position and is stretched by the tension of the stretching machine 13. The branching ratio measuring device 23 measures the branching ratio during the extension, and the Y-polarized light incident on the optical fiber OPF ₁ from the light source 19 enters the other optical fiber OPF ₂ at 100%.
% Or the first state branched to a state close to this is detected, and the stretching is stopped at that time. The stretched amount at this time was 8 mm. Since the wavelength λ ₁ of the Y polarized light incident from the light source 19 was 1.55 μm, the stretching amount was 10
It is about 5200 times the wavelength λ ₁ that realizes 0: 0 (%). The beam splitter manufactured under these conditions also exhibited the branching ratio characteristic shown in FIG.

A comparative example in which the conditions of the gas burner flame are the same as those in the above-mentioned embodiment, and the closest distance, the rest time and the stretching length are changed will be shown. Comparative Example 1 The stretching was stopped when the closest distance was 2 mm, the resting time was 1 second, and the stretching length was the second time when the branching ratio was 100: 0%. The branching ratio characteristic of the branching device thus obtained is shown in FIG. When this manufacturing condition is adopted, the branching ratio once reverses to 0: 100 (%) on the shorter wavelength side than the wavelength λ ₁ of the polarized light used for the measurement of the branching ratio during the stretching, and then 10
It exhibits the characteristic of returning to 0: 0 (%). After all, by counting how many times the branching ratio is inverted during stretching, the number of times the branching ratio changes on the shorter wavelength side than the wavelength λ ₁ can be specified. However, in this comparative example, the closest distance was set to 2 mm and the rest time was set to 1 second. Therefore, the optical fibers OPF ₁ and OPF ₂ were sufficiently heated and did not approach a completely melted state. It is considered that the film is stretched without any resistance, and therefore without increasing the internal stress. This is well illustrated by the branching characteristic shown in FIG. That is, it is understood that the branching ratio characteristic shown in FIG. 5 has a high wavelength dependency.
This means that the birefringence of the optical fibers OPF ₁ and OPF ₂ is lower than the birefringence of the beam splitter obtained in the above-described embodiment, especially in the fused portion. It can be seen that the beam splitting characteristics cannot be obtained because the phases of the Y polarization are close to each other. Comparative Example 2 The stretching was stopped at the time when the closest approach distance was 2.3 mm, the rest time was 1.2 seconds, and the stretching length was the third branch ratio of 100: 0 (%). The branching characteristics of the beam splitter thus obtained are shown in FIG. Even when this manufacturing condition is adopted, the fusion-bonded portion is sufficiently heated and the melting proceeds to a state close to a completely melted state, so that the stress generated during stretching is small.
That is, since it is stretched with a small force, the internal stress in the fused portion is not increased, and the birefringence is not increased, so that only the branch ratio characteristic having high frequency dependence can be obtained.

In the first and second embodiments described above, two optical fibers are used.
Although the case of using a polarization beam splitter has been described, as shown in FIG. 7, three polarization plane maintaining fibers can be fused to form a polarization beam splitter. When the three optical fibers are fused as described above, for example, a single polarized light is given to one end of the central optical fiber OPF ₂ , and this polarized light is one or both of the other two optical fibers. Further, if the stretching is stopped in the state of nearly 100% branching, branching characteristics similar to those in FIG. 3 can be obtained.

[0028]

As described above, according to the method of manufacturing the fiber fusion-stretched polarization beam splitter according to the present invention, it is possible to obtain the fiber fusion-stretched polarization beam splitter whose branch ratio characteristic has low frequency dependence. You can In addition, since it is possible to obtain the fiber fusion-stretched polarization beam splitter with low frequency dependence, even if the wavelength of the measured light fluctuates to some extent, the fluctuation of the wavelength does not appear as a change in the light quantity. Therefore, the actual benefit of realizing an optical measuring instrument with high measurement accuracy can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a method of manufacturing a fiber fusion-stretched polarization beam splitter according to the present invention.

FIG. 2 is a front view for explaining the structure of the main part of the manufacturing method shown in FIG.

FIG. 3 is a characteristic curve diagram showing a branching ratio characteristic of a fiber fusion-spreading type polarization beam splitter manufactured by the manufacturing method according to the present invention.

FIG. 4 is a cross-sectional view for explaining a cross-sectional structure of a fusion-bonded portion of a fiber fusion-stretched polarization beam splitter manufactured by the manufacturing method according to the present invention.

FIG. 5 is a characteristic curve diagram showing a branching ratio characteristic of a beam splitter when manufactured by a comparative example under conditions different from those of the manufacturing method according to the present invention.

FIG. 6 is a characteristic curve diagram similar to FIG.

FIG. 7 is a cross-sectional view showing a modified embodiment of the fiber fusion-spreading polarization beam splitter according to the present invention.

FIG. 8 is a diagram for explaining a practical example of a polarization beam splitter.

FIG. 9 is a side view for explaining a conventional technique.

FIG. 10 is a cross-sectional view for explaining the structure of a polarization-maintaining fiber.

FIG. 11 is a characteristic curve diagram for explaining a branching ratio characteristic obtained by a conventional manufacturing method.

FIG. 12 is a characteristic curve diagram similar to FIG.

[Description of Reference Signs] OPF ₁ and OPF ₂ Optical fiber 13 Stretching machine 14 Fixed clamp 15 Motor 16, 17 Heating means 18 Moving stage 19 Light source 21, 22 Photodetector 23 Branching ratio measuring device 24 Controller

Claims (4)

[Claims] 1. A clad having a core sandwiched between 18
It is composed of a plurality of polarization-maintaining fibers having a cross-sectional structure in which stress-applying portions are arranged at positions facing each other by 0 °, and these polarization-maintaining fibers are adjacent to each other and face each other. A part of the side edge is fused and the welded part is stretched in the axial direction to form light beams with polarization planes orthogonal to each other from one arm, and the two arms in the other arms respectively. A fiber fusion-stretched polarization beam splitter capable of beam-splitting and extracting light having one polarization plane and the other polarization plane, wherein the stretched amount of the welded portion is approximately 6000 times the wavelength of light to be beam-split. A fiber fusion drawing type polarization beam splitter characterized by being set as follows. 2. The core is sandwiched between 18
It is composed of a plurality of polarization-maintaining fibers having a cross-sectional structure in which stress-applying portions are arranged at positions facing each other by 0 °, and a part of side edges of the plurality of polarization-maintaining fibers that are opposed to each other are fused. , The light having two polarization planes orthogonal to each other is made incident from one incident side arm formed by extending the fusion-bonded portion in the axial direction, and one of the two arms of the outgoing side arms is polarized respectively. In a fiber fusion stretched polarization beam splitter that can separate and extract the light with the wavefront and the other polarization plane, the branching ratio does not change periodically on the shorter wavelength side than the wavelength of the light to be beam-split. A fiber fusion-spreading type polarization beam splitter comprising: 3. A polarization-maintaining fiber having a cross-sectional structure in which a stress-applying portion is arranged at a position facing each other by 180 ° with a core sandwiched in a clad having a circular cross-sectional shape. In the method for producing a fiber fusion-stretched polarization beam splitter formed by heating and fusing part of the side edges of the wavefront preserving fiber which face each other, and stretching the fusion-bonded part in the axial direction, While maintaining uniform tension on both polarization-maintaining fibers, the heating temperature is lower than the complete melting temperature of the optical fiber, and stretching is performed in the semi-molten state to increase the internal stress and increase the wavelength dependence. A method for manufacturing an optical fiber fusion-stretched polarization beam splitter, characterized in that a low optical fiber fusion-stretched polarization beam splitter is obtained. 4. The method for manufacturing an optical fiber fusion-stretched polarization beam splitter according to claim 3, wherein Y-polarized light is made incident from one end of a plurality of polarization-maintaining fibers, and a plurality of polarization planes are input. A branching ratio measuring device is connected to each of the other ends of the storage fibers, and the polarization light 100 incident from the ends of the fibers other than the fiber incident from the start of the stretching is 100
%, The above-mentioned stretching is stopped at the first time of observing the emission, and a method for manufacturing a fiber fusion-stretched polarization beam splitter.